In signal transmission in a high frequency region, it is usually desired to improve the transmission rate and to reduce noises, and studies are being made with respect to the board materials for flexible printed wiring boards, the wiring techniques, the circuit configurations, etc.
Heretofore, in a flexible printed wiring board made of a laminate having an electrical conductor layer and an electrical insulator layer, a polyimide resin excellent in heat resistance has been used as the electrical insulator layer. As a process for producing a laminate comprising a polyimide resin layer and an electrical conductor layer, the following three methods are employed. (1) A method wherein a polyimide resin film and a metal foil such as a copper foil are bonded via an adhesive layer, (2) a method wherein a metal layer is formed on a polyimide resin film by a method such as vapor deposition and/or metal plating, and (3) a method wherein a metal foil is coated with a polyimide resin precursor, then a polyimide resin is formed from the precursor by heat treatment, etc. to form a polyimide resin layer on the metal foil. However, laminates obtained by these methods had no adequate adhesion between the polyimide resin layer and the electrical conductor layer and thus sometimes led to operation failure of the circuit (e.g. JP-A-2004-1510).
For application to a high frequency region, a flexible printed wiring board employing a fluororesin excellent in a low dielectric property as the electrical insulator layer, has been proposed. However, as the fluororesin is poor in the adhesive property, it is necessary to use a heat resistant resin such as a polyimide resin to bond the fluororesin layer to the electrical conductive layer, whereby the low dielectric property of the fluororesin can not be adequately utilized (e.g. JP-A-2004-128361).
The adhesion between the electrical insulator layer and the electrical conductor layer, may be improved by forming irregularities of about 3 μm on the surface of the electrical conductor layer on the side being in contact with the electrical insulator layer, but due to a skin effect in a high frequency region, there will be a deviation in the signal arrival time between the surface having such irregularities (hereinafter referred to also as a roughened surface) and a non-roughened surface. Therefore, such irregularities are required to be profiled as low as possible (e.g. JP-A-5-55746). Here, the skin effect is a phenomenon such that a high frequency electrical current will flow only in the vicinity of the surface of the electrical conductor layer. For example, the electric current will flow only within a range of 2.3 μm in depth from the surface with 1 GHz or within a range of 0.7 μm in depth from the surface with 10 GHz.
Accordingly, it is desired to develop a flexible printed wiring board excellent in signal response in a high frequency region.